The present invention relates to a combined cycle power generation plant, and a method of operating such a plant, the plant comprising a boiler with a furnace for combusting a first fuel to produce steam, a steam generator driven by the steam to generate power and a combustor for combusting a second fuel to produce exhaust gas, which is expanded in a gas turbine to generate power and passed as a process gas to the furnace. The boiler is especially designed by taking into account the characteristics of the exhaust gas from the gas turbine as a process gas in order to render possible effective steam production with low emissions. The present invention addresses a problem of maintaining high performance of the boiler under varying operating conditions of the gas turbine combustor, including an operating mode in which the gas turbine combustor is not in use.
The combined cycle power generation plant may also comprise a gasifier to produce fuel gas and combustible char. The char may be used as the first fuel in the furnace of the boiler and the fuel gas as the second fuel in the gas turbine combustor. Thus, the plant can comprise a plurality of, such as two or three, separate systems, e.g., a boiler with a furnace, a gas turbine with a combustor and possibly a gasifier, which all are normally used together as an integrated system. The boiler is preferably a fluidized bed boiler, but it can also be a suspension boiler or some other type of boiler. The gasifier is preferably a pressurized fluidized bed gasifier, but it can also be of some other type. Both the gasifier and the combustor are preferably operated as circulating fluidized bed (CFB) systems.
U.S. Pat. No. 3,986,348 and No. 4,470,255 and Great Britain Patent No. 2,095,762 disclose combined cycle power plants, in which a pressurized gasifier, a gas turbine and a fluidized bed boiler are integrated so that fuel gas produced in the gasifier is combusted in a combustor of the gas turbine and the exhaust gases from the gas turbine are led to the fluidized bed boiler to function as a process gas, and char produced in the gasifier is combusted in the furnace of the fluidized bed boiler. In these types of plants, the particles in the product gas of the gasifier have to be removed before the gas is passed to the gas turbine, but most of the gaseous emissions can be removed at or downstream of the furnace of the CFB boiler, which renders possible cost-effective manufacturing and operation of the system.
In order to keep the emission level low, the amount of oxygen in the process gas has to be closely connected to the fuel feed rate. When compared to using air as combustion gas, the gas turbine exhaust gas is lean, including typically about 10 to about 15% by volume of oxygen, and hot, having a typical temperature of about 500 to about 600xc2x0 C. Thus, when using the gas turbine exhaust gas as combustion gas, the flow rate of the combustion gas is high, which has to be taken into account when designing the boiler. Generally, the cross-sectional area of the furnace has to be large, the means for supplying process gas, e.g., the grid of a fluidized bed boiler, has to allow a high gas flow rate, and more heat transfer surfaces than normal have to be located in the back-pass of the boiler.
In these kinds of systems, the quantity and quality of the exhaust gas may strongly depend on the operating conditions of the gas turbine combustor. Thus, without special precautions, the performance of the boiler may vary under different operating conditions of the gas turbine combustor, and the efficiency of the system and the emissions released to the environment may, in some conditions, be far from optimal.
There may be a need to run the system in different operating modes, e.g., when having the gasifier down because of regular maintenance. The power should then be generated, e.g., by means of the boiler system alone, without having gas turbine exhaust gas available. If, under such operating conditions, fresh air is used as the process gas in the boiler, it may be impossible to gain optimal or even acceptable performance. By using optimal process gas flow, good bed temperature can be achieved, but, on the other hand, high excess air is produced, which results in a low boiler efficiency and high NOx emissions. Another alternative would be to use low excess air, but that would in turn lead to too high a bed temperature and very high SO2 emissions.
It is an object of the present invention to provide a combined cycle power generation plant, including a gas turbine combustor and a boiler, which can provide high performance such as high efficiency and low emissions under (i) varying operating conditions of the combustor or (ii) an operating condition in which the combustor is not in use. It is also an object of the present invention to provide a method of using such a power generation plant.
In order to achieve these and other objects of the present invention, a combined cycle power generation plant and a method of using such a power generation plant are provided, as described in the independent claims.
In one aspect, the present invention provides a method of operating a combined cycle power generation plant that includes providing a boiler having a furnace for combusting a first fuel at a first temperature to produce flue gas and for producing steam, the boiler having an optimal performance in terms of steam production and emissions to the environment, wherein the first temperature provides an optimal temperature, conducting the flue gas through a flue gas duct to the environment, supplying process gas to the furnace at a first mass flow rate, the first mass flow rate providing an optimal mass flow rate, driving a steam turbine by the steam to generate power, combusting, in a combustor, a second fuel to produce exhaust, expanding the produced exhaust gas in a gas turbine to generate power, passing the exhaust gas from the gas turbine to the process gas supply, recirculating a portion of the flue gas from the flue gas duct to the process gas supply, controlling the rate of recirculation of the flue gas by a first controller, supplying fresh air to the process gas supply, controlling the rate of fresh air supply by a second controller, supplying a selected amount of first fuel to the furnace, supplying a selected amount of second fuel to the combustor, and the boiler having, in first operating conditions of the combustor, the optimal performance when the first and second controllers minimize the rate of flue gas recirculation and fresh air supply, respectively, and the exhaust gas is used alone or as a major portion of the process gas, and controlling in conditions other than the first operating conditions of the combustor, the first and second controllers to obtain at least nearly the optimal performance of the boiler.
In another aspect, the present invention provides a combined cycle power generation plant that includes a boiler for producing steam, the boiler including a furnace for combusting a first fuel to produce flue gas, a back-pass for receiving the produced flue gas and a flue gas duct for passing the flue gas from the back-pass to the environment, a process gas supply for supplying the process gas to the furnace, a steam turbine for receiving and being driven by the steam to generate power, a combustor for combusting a second fuel to produce exhaust gas, a gas turbine for expanding the exhaust gas from the combustor to generate power and for passing the exhaust gas to the process gas supply, a return line for recirculating a portion of the flue gas from the flue gas duct to the process gas supply, a first controller for controlling a rate at which the flue gas is recirculated in the return line, a supply for supplying fresh air to the process gas supply, a second controller for controlling a rate at which the fresh air is supplied by the supply, and a controller for controlling the first and second controllers so as to maintain at least nearly optimal performance of the boiler under different operating conditions of the combustor. The boiler has a geometry and heat transfer surfaces located so as to optimize performance in terms of steam production and emissions to the environment, when using exhaust gas alone or as a major portion of the process gas being supplied to the boiler.
It is assumed above that the first operating conditions correspond to normal operating conditions of the system, which are used as the basis when designing the system. In these conditions, unmixed gas turbine exhaust gas, or exhaust gas mixed with a small amount of air and recirculated flue gas, typically at most about 10% by volume, is used as process gas in the furnace of the boiler. The typical flow rate and oxygen content of the exhaust gas are taken into account when designing the geometry of the boiler and the location of the heat transfer surfaces within the boiler. Because the characteristics of the exhaust gas may vary under different operating conditions of the gas turbine combustor, a primary goal of the present invention is to provide a method and a system that compensate for the changes of the characteristics of the process gas supplied into the boiler under varying operating conditions of the gas turbine combustor. According to a preferred embodiment of the present invention, this goal is achieved by controlling the supply of fresh air to the lower portion of the boiler and the recirculation of flue gas from the flue gas duct into the lower portion of the boiler, so that under varying operating conditions of the combustor, optimal performance of the boiler, in terms of steam production and emissions to the environment, is obtained.
Usually, air is used as a fluidizing gas and process gas of a fluidized bed combustor. The amount of air introduced into the combustor is determined on the basis of the amount of oxygen needed for the combustion. U.S. Pat. No. 4,355,601 and No. 4,441,435, however, disclose mixing of the fluidizing gas with an amount of recirculated flue gas in order to control, e.g., the bed temperature or residual O2 in the flue gas, when the boiler load or the characteristics of the fuel introduced into the boiler is changed. The embodiment of the present invention described above differs from the previously known forms of flue gas recirculation in that, in the present case, the need for controlling the process gas is not due to something directly affecting the boiler, but something primarily affecting the gas turbine combustor of the power plant, and influences the boiler only through the gas turbine exhaust gas. More specifically, the present boiler is designed to be used with lean exhaust gas as the process gas, and the supply of fresh air and the recirculation of flue gas are performed in order to maintain the characteristics of the process gas optimal under different conditions.
An example of using the present invention is to regulate the mass flow of the process gas in varying ambient air temperatures. It is typical for gas turbine systems for the volume flow rate of the inlet air to be nearly constant under different conditions. This means that the mass flow rate, and also the amount of oxygen, in the exhaust gas of such systems may be at a high ambient temperature clearly lower than that at lower temperatures. According to the present invention, it is possible to compensate for the decreasing exhaust gas flow by providing a proper amount of fresh air and recirculated flue gas.
According to a preferred embodiment of the present invention, the power generation plant may also comprise a high pressure gasifier for gasifying a third fuel to produce fuel gas, which can be used as the second fuel. The gasifier is preferably a fluidized bed gasifier, fluidized with air. An efficient power generation cycle is provided when char, which is a combustible residue of the gasification process, is used for fueling a fluidized bed boiler.
The changes of the operating conditions of the gas turbine combustor may be gradual changes of the operating environment or more drastic changes of the operating conditions. The conditions affecting the operation of the combustor, e.g., ambient air temperature, pressure and/or humidity, can be measured, and the controlling of the first and second controllers can be based on the measured conditions.
A sudden change in the operating conditions takes place, e.g., when the fuel used in the gas turbine combustor is changed to another or the total plant is switched into another operating mode. The biggest possible change is from a mode in which the gas turbine combustor is in operation to another mode in which the gas turbine combustor is not used. After such a change, the entire process gas has to be produced from fresh air and recirculated flue gas. Then, according to the present invention, the supply of fresh air and the recirculation of flue gas are controlled so as to simulate the characteristics of the original process gas. However, when using a mixture of recirculated flue gas and fresh air instead of gas turbine exhaust gas as the process gas, the temperature of the process gas is several hundreds of degrees Celsius lower than originally. If an amount of steam production is required to be unchanged, the fuel feeding rate to the furnace has to be increased, and the required amount of oxygen changes correspondingly.
If the combined cycle power generation plant comprises a gasifier, it is possible to change the operating mode from a mode including the use of the gasifier for producing the fuel to the gas turbine combustor to another mode in which a gasifier is not used, but instead, natural gas, e.g., is used for fueling the gas turbine combustor. In this case, the change of the process gas is not necessarily very large. Nevertheless, according to the present invention, the change of the characteristics of the process gas can be reduced.
According to a preferred embodiment of the present invention, the characteristics of the process gas and/or the performance of the boiler is/are measured, and the flow rates of the components of the process gas are controlled on the basis of at least one of these measurements. As an example, the oxygen content of the flue gas or the oxygen content and flow rate of the process gas can be measured and used as the basis of the control.
An actual goal in controlling the boiler is to achieve optimal performance of the boiler in terms of steam production and emissions to the environment.
A principal factor affecting the boiler performance is the combustion temperature. Thus, it is possible to measure a temperature in the boiler, e.g., the temperature in the lower portion of the furnace, and use the measured temperature as the basis for controlling the first and second controllers. Correspondingly, one can use measurements of emissions released to the environment as a basis for the initial adjustment of the process gas control. Also, the pressure, flow and/or temperature of the produced steam can be used for controlling the flow rates of fresh air and recirculated flue gas.